The present invention relates, generally, to motor vehicles equipped with electric power steering. This invention relates, more particularly, to a sophisticated method of controlling the thermal protection for the electric motor of such a power steering system.
In a power steering system of this type, an electric motor is used to apply a torque or a force to the steering column or to the steering pinion or even to the rack, to assist the movement imparted on the steering wheel by the driver of the vehicle, in each change of direction, parking or avoidance maneuver. When the power steering is required, an electric current of relatively high intensity passes through the windings of the electric assist motor.
Thus, in a long journey punctuated by turns, the electric power steering motor can become hot. If the heating becomes intense and prolonged, there is a risk of damaging, or even destroying, the electric assist motor.
To avoid this risk, thermal protection is provided, in a known manner, to protect the motor against excessive overheating. In particular, depending on parameters, some of which are supposed to describe the thermal state of the motor, a maximum current that can be sent to feed this motor, to drive it, is calculated at each instant.
More particularly, at each instant, depending on the torque applied manually to the steering wheel by the driver, an assist current is calculated to feed the electric assist motor, in order to assist the movement imparted by the driver in optimum conditions. When this calculated current is greater than the maximum current determined elsewhere, the set point current of the motor is chosen to be equal to the maximum calculated current.